Multi-Clause Sentencesbelieve that the reason has to do with the discourse functions of subjects.
Typically, though by no means always, subjects function as the topics of
their sentences. Topics refer to the entities that the sentences are about. So
(78b) is about Oscar in a way that (78a) is not; similarly, (80b) is about
Hilda, whereas (80a) is not. We would use the (b) sentences of these pairs
in discourse contexts slightly different from the contexts in which we would
use the (a) versions.
We turn now to a pair of sentence types that have characteristics akin to
finite relative clauses, the it-cleft and pseudo-cleft constructions.
It-clefting
The following is an it-cleft sentence, and we will refer to the phrase in square
brackets as its focus and to the italicized clause simply as its clause:
(81) It was [Henry Ford] who invented the assembly line.Clefts consist of an expletive it higher subject, a form of be, a focus phrase
(which may be any phrase type except VP), and a clause that looks like (but
actually isn’t) a finite relative clause.
The clause is like a relative in that it may be introduced by a wh-word, that,
or (in some cases) nothing at all:
(82) It was Henry Ford (who/that/zero) invented the assembly line.It also contains a “gap,” which is interpreted as if it were “filled” by the focus
phrase, so that the clefts above mean in essence:
(83) Henry Ford invented the assembly line.The fact that a cleft can be reduced in this way has led some grammarians to
suggest that the focus was actually moved out of the clause into its position in
the higher clause.
Cleft foci are often interpreted as contrasting with some other phrase. For
example, you might use a cleft such as the ones above if you thought that the
audience believed that Roger Smith invented the assembly line:
(84) It was Henry Ford, not Roger Smith, who invented the assembly
line.If the focus is a PP, then the sense of contrast may fade somewhat: